A semiconductor wafer of, for example, silicon or gallium arsenide is produced in the form of a large diameter. This wafer is cut and separated (diced) into small element chips and is subjected to a subsequent mounting step in which the chips are mounted on lead frames. In this process, the semiconductor wafer undergoes dicing, cleaning, drying, expanding and pickup steps in the state of being attached to a pressure sensitive adhesive sheet, and transferred to the subsequent mounting step.
Adhesive sheets which are intended for use in the processing steps of wafers, from the dicing step up to the pickup step, are desired to have an adhesive force sufficient to retain the wafer and/or chips thereon during the dicing step, but in the pickup step, they are desired to only retain an adhesive force of such an extent that no adhesive remains on the picked-up wafer and/or chips.
In the efforts for obtaining such a desired pressure sensitive adhesive sheet, Japanese Patent Laid-open Publication Nos. 60(1985)-196956 and 60(1985)-223139 proposed pressure sensitive adhesive sheets each comprising a substrate coated with a pressure sensitive adhesive comprising a low molecular weight compound having at least two photopolymerizable carbon-carbon double bonds in each molecule thereof which can be converted to a three-dimensional netted structure by light irradiation. These proposals are based on the concept that, in a pressure sensitive adhesive tape comprising a radiation transmittable substrate and, applied thereon, a radiation curable pressure sensitive adhesive, the radiation curable compound contained in the pressure sensitive adhesive is cured by radiation irradiation so that the pressure sensitive adhesive comes to have a three-dimensional netted structure to thereby conspicuously lower the fluidity thereof. However, in the pressure sensitive adhesive sheets described in the above publications, the curing reaction is likely to cause the pressure sensitive adhesive layer to suffer from loss of the rubber elasticity, so that satisfactory extension cannot be attained in the expansion of the sheet. Therefore, the spacings between wafer chips neighboring each other are not satisfactory and not uniform to thereby cause operation error at the pickup step.
Japanese Patent Laid-open Publication No. 5(1993)-214298 teaches a pressure sensitive adhesive sheet provided with a pressure sensitive adhesive layer having, added thereto, a vinyl ether compound of relatively low molecular weight for maintaining the rubber elasticity of the adhesive layer and an unsaturated oligomer of relatively low molecular weight for rendering the adhesive layer curable by radiation irradiation. Further, Japanese Patent Laid-open Publication No. 6(1994)-49420 discloses a pressure sensitive adhesive sheet provided with a pressure sensitive adhesive layer having, added thereto, a polyfunctional urethane acrylate oligomer and a plasticizer such as a polyester compound. However, when the amount of added low molecular weight component or plasticizer is too large, this pressure sensitive adhesive suffers from lowering of its initial adhesive strength, so that the drawback would be encountered that the low molecular weight component or plasticizer remains on the back of the semiconductor wafer chips. On the other hand, when the amount of added low molecular weight component or plasticizer is too small, the drawback would be encountered that the reduction of the adhesive strength is unsatisfactory or the rubber elasticity cannot be obtained upon radiation irradiation Therefore, the control of adherent properties has been very difficult before and after the radiation irradiation.
The present invention provides a pressure sensitive adhesive sheet which has satisfactory pressure sensitive adherence and initial adhesion before the irradiation with energy beam but whose adhesive strength is sharply reduced after the irradiation with energy beam with maintaining the rubber elasticity, this pressure sensitive adhesive sheet ensuring excellent chip alignability in the expanding step subsequent to dicing. The present invention further provides a pressure sensitive adhesive composition which can form the above pressure sensitive adhesive sheet.